The present invention relates to an apparatus for controlling a pressure and a flow rate in which even when a primary pressure or a secondary pressure to a variable throttle for adjustment of a flow rate varies, a front-to-back differential pressure of the variable throttle is always held to be constant and a constant flow rate which is set depending upon a throttle opening is applied to a load, and at the same time a secondary pressure is also controlled.
Hitherto, in case of controlling both of a flow rate and a pressure, a flow control valve FV and a pressure control valve PV are respectively used as a combination as shown in FIG. 1.
However, the flow control valve FV and pressure control valve PV themselves are large-sized, respectively. In particular, the flow control valve has a drawback such that a flow rate of a fluid passing through the valve changes in the case where there is a variation in front-to-back differential pressure .DELTA.P of a variable throttle 2; therefore, a differential pressure regulator is combined as will be described later. Thus, an increase in size of the valve itself cannot be avoided.
A variation in flow rate of a fluid which passes through the variable throttle due to the front-to-back differential pressure will be first considered. When it is assumed that a flow rate of the fluid passing through the throttle is Q, a flow coefficient in the throttle portion is c, an area of the opening of the throttle is A, and a density of the fluid is .rho., the flow rate Q will be given by the following equation. ##EQU1## It will be understood from equation (1) that the flow rate Q varies in dependence upon the front-to-back differential pressure .DELTA.P of the throttle.
Therefore, in the conventional flow control valve FV, on the basis of the principle such that the flow rate does not change when the front-to-back differential pressure .DELTA.P of the throttle is set to be constnt from the foregoing equation (1), the variable throttle 2 and a differential pressure regulator 4 are combined and the front-to-back differential pressure of the throttle is made constant even if there is a variation in pressure before and after the throttle, thereby keeping the flow rate of the fluid which passes through the valve to become a set flow rate.
Practically speaking, in the flow control valve FV in FIG. 1, the differential pressure regulator 4 is connected in series to an oil passage on the side of the inlet of the variable throttle 2, an inlet pressure P.sub.1 of the variable throttle 2 is led into a primary pilot chamber of the differential pressure regulator 4, and an outlet pressure P.sub.2 of the variable throttle 2 is led into a secondary pilot chamber equipped with a differential pressure setting spring.
A further detailed explanation will be made with reference to a structure of the valve in FIG. 2. A pressure compensating spool 8 is slidably arranged in the passage from the inlet to the outlet of a body 6. A pressure compensating orifice 10 is formed between the land on the right side of the spool 8 and the inlet passage. A primary pilot chamber 14 into which the inlet pressure P.sub.1 of the variable throttle 2 is led is formed on the right side of the spool 8, while a piston 16 of a large diameter is integrally formed on the left side. The inlet pressure P.sub.1 is led onto the right side of the piston 16 of a large diameter. Also, a differential pressure setting spring 20 is provided in a secondary pilot chamber 18 on the left side of the piston 16. The outlet pressure P.sub.2 of the variable throttle 2 is led into the secondary pilot chamber 18.
For the operation of the flow control valve FV shown in FIGS. 1 and 2, the fluid enters from the inlet and passes through the pressure compensating orifice 10 and variable throttle 2 and reaches the outlet. In this case, the inlet pressure P.sub.1 of the variable throttle 2 acts on area portions A.sub.1, A.sub.2 and A.sub.3 of the pressure compensating spool 8 through a small hole and the outlet pressure P.sub.2 acts on the area portion A.sub.1. Therefore, when considering the balance of the forces which act on the pressure compensating spool 8 in the stationary state whereby the fluid is flowing, EQU F+A.sub.1 .times.P.sub.2 =(A.sub.2 +A.sub.3).times.P.sub.1
(where, F is a compression force of the differential pressure setting spring 20).
Since A.sub.1 =A.sub.2 +A.sub.3, we have EQU P.sub.1 -P.sub.2 =F/A.sub.1 ( 2)
Thus, it will be understood from equation (2) that the front-to-back differential pressure (P.sub.1 -P.sub.2) of the variable throttle 2 becomes constant.
Practically speaking, when an inlet pressure P.sub.0 varies, an inflow amount from the pressure compensating orifice 10 changes in dependence upon the pressure P.sub.0 and the front-to-back differential pressure of the variable throttle 2 also changes, so that the forces which act on the pressure compensating spool 8 become unbalanced. Namely, when the inlet pressure P.sub.0 increases, the pressure compensating spool 8 is moved to the balanced position on the left side. On the contrary, when the inlet pressure P.sub.0 decreases, the pressure compensating spool 8 is moved to the balanced position on the right side.
On one hand, in the case where the outlet pressure P.sub.2 changes as well, the forces which act on the pressure compensating spool 8 become unbalanced, so that the spool 8 is moved to the balanced position on the left side with a decrease in outlet pressure P.sub.2, while the spool 8 is moved to the balanced position on the right side with an increase in outlet pressure P.sub.2.
Thus, due to the operation of the pressure compensating spool 8, the differential pressure regulator 4 functions such that F/A.sub.1 in the foregoing equation (2) becomes constant, thereby making the passing flow rate of the variable throttle 2 constant.
However, in such a conventional flow control circuit, the flow rate and pressure of the pressure compensating orifice 10 of the differential pressure regulator 4 also change in dependence upon the pressure and flow rate of the fluid flowing through the variable throttle 2. These changes influence the balance based on the differential pressure setting spring 20 of the pressure compensating spool 8, so that the front-to-back differential pressure of the variable throttle 2 does not become a stable constant value.
To solve such a problem, there is considered a method whereby the hydraulic acting areas A.sub.1, A.sub.2 and A.sub.3 of the pressure compensating spool 8 are enlarged and a spring force of the differential pressure setting spring 20 is also made large. However, there are the following problems. Namely, even if the use flow rate (operating flow rate of the fluid which is practically applied) is the same, the control valve increases in size. The response speed also deteriorates in association with an increase in size of the spool. Further, when the differential pressure setting spring is made strong, the lowest working pressure upon operation of the differential pressure regulator increases.
On the other hand, the reduction in response speed can be solved by enlarging the area of the passage of the pilot passage; however, this obviously causes a problem such that the valve is increased in size.